Efeito da temperatura de calcinação em catalisadores de platina a base de óxidos de cério e zircônio mássicos e suportados em alumina nas reações de oxidação parcial e reforma autotérmica do metano

Abstract

The transformation of methane into higher value products became more important in recent years due to the search for new energy sources. The use of partial oxidation and autothermal reforming of methane can give synthesis gas with a H2/CO ratio of 2, that is very adequate for the Fischer-Tropsch synthesis. In order to develop this process, the use of catalysts with high activity, stability and selectivity even when submitted to high temperatures is necessary. The objective of this study was to evaluate the behavior of platinum catalysts impregnated on ceria and ceria-zirconia oxides, both unsupported and supported on alumina for the reaction of partial oxidation and autothermal reforming of methane. The samples prepared at 1073 K were considered fresh and the catalytic and physical properties for these samples as well as the ones calcined at 1173 and 1273 K were investigated. Ceria and ceria-zirconia carriers were prepared by precipitation and ceria and ceria-zirconia supported on alumina containing 11 and 14 wt% of CeO2 or CeZrO2 were obtained by dry impregnation. After this platinum was added by dry impregnation with a loading of 1.5 wt% for unsupported and supported samples containing 14 wt% of ceria or ceria-zirconia oxides. The samples with 11 wt% of ceria based material on alumina were impregnated with 1.2 wt% of platinum. The catalysts containing zirconia showed higher values of BET surface area, oxygen storage capacities (OSC) and metal dispersion than the ones containing only ceria. They also were more resistant to the exposure to higher calcination temperatures. The temperature programmed reduction (TPR) profiles demonstrated a greater reducibility of ceria-zirconia samples which is in agreement with the OSC results. The X-ray diffraction data showed that probably a ceriazirconia solid solution was formed and that the samples containing zirconia were more resistant to the particle growth with the increase of calcination temperature. The catalytic tests during the partial oxidation of methane reaction for 1.5%Pt/Ce0.5Zr0.5O2, 1.2%Pt/11%CeO2/Al2O3, 1.2%Pt/11%Ce0.5Zr0.5O2/Al2O3 and 1.5%Pt/14%Ce0.5Zr0.5O2/Al2O3 showed that, in general, the samples supported on alumina were more active than the samples supported on the ceria based oxide for all calcination temperatures. Some samples presented a slight deactivation after 24 hours of reaction and an increase of formation of CO2, specially 1.5%Pt/Ce0.5Zr0.5O2 and 1.5%Pt/14%Ce0.5Zr0.5O2/Al2O3 calcined at higher temperatures. The autothermal reforming of methane tests showed that 1.5%Pt/14%Ce0.5Zr0.5O2/Al2O3 were more active and stable than 1.5%Pt/14%CeO2/Al2O3 catalysts. This was attributed to better redox properties of these samples.